Genomic instability is a characteristic of cancer cells. Homologous recombination (HR)-mediated DNA repair represents an error-free repairing mechanism to maintain genomic integrity and ensure high-fidelity transmission of genetic information. My long-term goal is to establish a successful and sustainable independent research program with a core competency in the study of HR repair using genomic and proteomic approaches. In my independent career, I wish to pursue (1) fundamental studies to understand HR-mediated DNA repair and its dysfunction in tumorigenesis and (2) mechanism-based translational studies to translate fundamental breakthroughs in HR repair into clinical applications in cancer prevention, diagnosis, prognostication, and therapy. The overall objective of my proposed research, which will lay the foundation for my independent research career, is to understand the novel nuclear function of human Ago2, a core protein in RNA interference pathways, in HR repair and genome maintenance. Based on my preliminary data, I hypothesize that in the context of DNA damage response, via fine-tuned regulatory mechanisms by posttranslational modifications, potentially through ATM/ATR kinase-dependent phosphorylation, human nuclear Ago2 regulates HR repair of double-strand breaks by recruiting DNA repair proteins at damage sites via protein-protein interactions. I will test this hypothesis by pursuing 3 specific aims through an integrated platform that combines mechanistic and functional studies: (1) Determine the function of Ago2 as a novel regulator in HR repair. (2) Characterize posttranslational modifications of Ago2 induced by DNA damage in HR repair. (3) Determine the nuclear function of Ago2 in preventing genomic instability and tumorigenesis. The proposed research is significant because it challenges the current research paradigm that human Ago2 functions predominantly in the cytoplasm. This study will shed light on how 2 evolutionarily conserved genome defense mechanisms, the small regulatory RNA pathways and DNA damage response pathway, converge at DNA lesions in the process of HR repair via the functional involvement of Ago2 protein.